The present exemplary embodiments relate to a method for forming ceramic (e.g., piezoelectric such as lead zirconate titanate (PZT)) thick film element arrays with fine feature size, high-precision definition, and/or high aspect ratios. It finds particular application in conjunction with high frequency and/or ultrasonic implementations, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Ceramic thick films, especially piezoelectric thick film (thickness between 10 to 100 μm) element arrays with fine feature size, high-precision definition, and/or high aspect ratios have many applications in, for example, micro electromechanical systems (MEMS) devices, nondestructive testing and medical imaging. For example, as shown in FIG. 1, a one-dimensional (1-D) piezoelectric element array 10 for medical ultrasound imaging may comprise a plurality of high-aspect elements 12. To operate the piezoelectric element array 10 at the desired resonant frequency, i.e., that of the longitudinal mode, to obtain maximum acoustic power output, the thickness t of the piezoelectric elements should be approximately λPZT/2, where λPZT is the acoustic wavelength in PZT ceramics. As the sound velocity of PZT ceramics is about 4000 M/s and a typical frequency used in the present technology is 40 MHz, the thickness of the PZT elements should be approximately 50 μm. Or the other hand, in order to reduce the side lobe of the waveform, the center-to-center distance d of the PZT elements should be about half of the acoustic wavelength in human organs, which is close to that of water. This results in the lateral dimension b of the PZT elements being approximately λwater/3. Considering λPZT≈(4/1.5) λwater, the height-to-width aspect ratio of the PZT elements should be at least 4/1.5)/2/(⅓)=4. Thus, for the 50 μm-thick PZT films, the lateral dimension b should be about 12.5 μm, and the spacing between the elements should be about 6.25 μm.
At present, there is a need for cost effective methods to directly form piezoelectric elements with a thickness between 10 to 100 μm, such as 50 μm. Moreover, there is a need for effective methods for forming arrays of elements with a high aspect ratio, as described above, in such a thickness range. Attempts to satisfy these needs are outlined below.
A method has been proposed for producing high aspect ratio PZT thick film elements by combining a sol-gel technique with SU-8 molding. N. Futai, K. Matsumoto, I. Shimoyama, “Fabrication of High-Aspect-Ratio PZT Thick Film Structure Using Sol-Gel Technique and SU-8 Photoresist,” Technical Digest of MEMS 2002 IEEE International Conference, pp. 168-171 (2002). However, there is quite a large portion of organics in the sol-gel derived PZT elements, which needs to be burned out during sintering or annealing. This leads to the difficulty of densifying high-aspect-ratio sol-gel PZT films made by using a SU-8 mold, and thus affects the quality of the films. Second, due to the limited doping effect in the sol-gel films, the properties of the sol-gel derived PZT films cannot be easily tailored. That is, it is very difficult to make sol-gel PZT films to be intentionally soft or hard. Third, the sol-gel derived PZT films need to be annealed at 600° C. or higher, so the choice of the substrates is limited.
Others have proposed making 1-3 PZT/epoxy composites using soft molds and slip casting. S. Gebhardt et al., “Fine Scale Piezoelectric 1-3 Composites by Soft Mold Process: Preparation and Modeling,” Ferroelectrics, v. 241, pp. 67-73 (2000). However, it is difficult to use these methods for elements having a very fine feature size, such as less than 10 μm in the lateral dimension or in spacing.
In U.S. Pat. No. 7,089,635 B2, bearing application Ser. No. 10/376,544, and Publication No. 2004/0164650 A1, filed on Feb. 25, 2003, entitled “Methods to Make Piezoelectric Ceramic Thick Film Array and Single Element and Devices,” and naming Baomin Xu et al as inventors, a technique combining screen printing and laser liftoff has been proposed to make high quality piezoelectric ceramic element arrays on almost any kind of substrate. This method can provide high quality PZT elements because the PZT elements are sintered on a sapphire substrate and, thus, there is no limitation on sintering conditions. Also, for the final target substrate, this is a low-temperature and clean process because the PZT elements are transferred to the final target substrate after sintering. The cost of this method is also much lower than sol-gel processing because it uses conventional bulk raw materials and the sapphire substrate can be re-used.
In U.S. Pat. No. 7,070,669 B1, bearing application Ser. No. 11/017,325, filed Dec. 20, 2004, entitled “A Method for Forming Ceramic Thick Film Element Arrays,” and naming Baomin Xu and Stephen David White,  an alternative transfer technique has been proposed. That is, the printing substrate is advantageously provided with a release layer, and making it possible to release the printed and soft-baked ceramic elements from the printing substrate and transfer them to the sintering substrate. After sintering, the ceramic elements are transferred to the final target substrate.
However, there are several qualities of some screen printing methods that do not render them particularly advantageous when applied to high aspect ratio and/or very fine feature size implementations. First, it is difficult to make high thickness-to-width aspect ratio elements. Second, the smallest feature size and spacing for screen printing is about 50 μm, but many MEMS devices may require piezoelectric elements with dimensions smaller than 50 μm. Third, the edge of screen printed elements is not very sharp and there is typically a transition area along the edge.